Dependent steering axle suspensions have been consistently used in virtually all commercial trucks and buses. Dependent steering axle suspensions physically link right and left steered wheels by a solid cast iron bar (axle) under the front of the vehicle. The bar is suspended from the chassis by heavy leaf springs and shock absorbers.
Such a design is inherently subject to conditions that include but are not necessarily limited to: axle oscillation when one front wheel hits a bump; gyroscopic torque about the steering axis that tends to turn the axle from left to right when the vehicle is being forwardly propelled; axle inertia amplification of original motion from a road bump; substantial unsprung weight due to a heavy bar that requires heavy linkages and leaf springs (i.e., unsprung weight) for its suspension; wheel misalignment if the axle becomes bent; and poor, or non-existent, lateral control of axle bar location, a significant performance issue.
The internal combustion engine, typically a diesel engine, has consistently been the powerplant of large commercial vehicles. However, alternate types of powerplants are gaining more widespread usage in the broader automotive industry for various reasons, primarily emission- and fuel-related.
One type of alternate-powered vehicle has electric drive motors mounted directly into the wheels. Such a propulsion system however significantly increases the ratio of unsprung to sprung weight of the vehicle. Sprung weight comprises the weight of that portion of a vehicle supported by the springs, and unsprung weight comprises the weight of the springs and that of the remainder of the vehicle. Wheel mounted motors are also subject to severe impacts from road bumps during service, and such impacts can significantly shorten the life of these electric drive units.
In one hybrid commercial vehicle, an example of which is the Orion VII Hybrid Transit Bus, an electric motor is coupled to the output of a diesel engine and is mechanically coupled to a heavy, rigid, conventional commercial truck rear axle. The weights of both the electric motor and the diesel engine are sprung because they are chassis-mounted. However, the rear suspension in this vehicle is a heavy, dependent (i.e., non-independent) system, and the axle is a non-steering axle.
In a de Dion tube suspension, the differential and the disk brakes are removed from the unsprung weight by hanging them to the chassis. Such a design is sometimes called chassis-mounted differential and brakes.